Pneumatic braking systems are commonly used on large commercial vehicles, such as trucks, trailers and buses. The considerable inertial mass of these heavy-duty vehicles in combination with the high speeds at which they travel requires a braking system which responds rapidly with substantial braking power. One system component of pneumatic braking systems is the spring brake actuator. The brake actuator provides the force necessary when braking the vehicle. There are several types of spring brake actuators including piston type spring brake actuators and diaphragm spring brakes. The service chamber of the spring brake actuator includes a housing defining a pneumatic chamber connected to the pneumatic system of the vehicle, a piston or push rod having a head or plate which reciprocates within the housing under pneumatic pressure and a return spring. In a diaphragm type spring brake actuator, a cup-shaped flexible diaphragm is supported within the chamber and the push rod plate is received against the diaphragm, such that one end of the push rod is driven through an opening in the housing when the pneumatic pressure flexes the diaphragm. A dual diaphragm spring brake actuator of the type disclosed herein includes an upper emergency chamber and a lower service chamber. The emergency chamber includes a power spring which is normally held in a compressed state by pneumatic pressure acting against the diaphragm in the upper chamber. When the pneumatic system of the vehicle fails or the parking brakes are applied, the power spring expands, driving the diaphragm and push rod in the upper chamber to brake the vehicle. The service chamber includes a second diaphragm and push rod assembly which actuates the braking system of the vehicle. Such spring brake actuators are well known in the art.
The push rod is normally secured to the end plate or end plates by a convention screw. The end plate includes an opening therethrough and the end of the push rod includes an internally threaded bore which receives the screw. However, there are several problems associated with the present method of attaching the end plate to the push rod. First, the connection between the push rod and the end plate must not loosen during use, particularly under the extreme temperature variations and vibration requirements. Second, the overall length of the push rod and plate assembly must be accurately controlled to the control the stroke of the push rod. Thus, the plate must be rigidly secured to the push rod. This is presently accomplished by using a thread adhesive such as Loctite.RTM. on the threads of the screw. As will be understood, however, applying a thread adhesive in a blind tapped hole cannot be fully controlled, particularly where the thread adhesive is applied by hand, and therefore at least some of the installations will fail. Further, non-destructive testing cannot assure that there will not be failures in the field. Finally, the exterior surface of the cylindrical push rod in the emergency chamber must be very accurately machined and coated with a hard protective coating to avoid leakage because of the pressure differential between the emergency and service chambers. A seal is provided between the emergency and service chambers and the push rod reciprocates through the seal. Therefore, any lack of concentricity or damage to the exterior surface of the push rod in the emergency chamber of a dual diaphragm spring brake actuator will result in leakage between the chambers. In the disclosed embodiment of the dual diaphragm spring brake actuator, an end plate is attached to both ends of the push rod. It is not therefore practical to forcefully grip the push rod of the emergency chamber during the connection of the end plates to the push rod which would cause deformation damage to the push rod and leakage between the chambers.
There is therefore a longstanding need to develop an effective method of attaching the end plate or end plates to the push rod of a spring brake actuator which assures a rigid permanent interconnection and which does not loosen during use. Further, a method of interconnecting the end plate to the push rod cannot result in damage to the push rod, particularly the push rod in the emergency chamber to avoid leakage between the emergency chamber and the service chamber. Finally, the method of attaching the end plate to a push rod cannot significantly increase the cost of the assembly. The improved brake actuator, particularly the improved push rod and plate assembly and method of attaching the end plate to a push rod of this invention accomplishes these objects.